A resonance circuit including a coil, a capacitor, and a circuit element equivalent to them is used in various electronic circuits. The resonance circuit is often required to have a resonance frequency controlling function. A resonance frequency of the resonance circuit is typically controlled by changing either one or both of an inductance value of the coil and a capacity (capacitance) value of the capacitor. As one of many important electric circuits utilizing a resonance behavior of the resonance circuit, an oscillation circuit and a filter have been known. The oscillation circuit and the filter are electronic parts indispensable for operating, for example, a portable telephone and various communication apparatuses. In addition, the oscillation circuit and the filter are often required to have an oscillation frequency controlling function and a frequency characteristic (a passband frequency characteristic and a stopband frequency characteristic), respectively.
The oscillation circuit and the filter are typically provided with a piezoelectric vibrator. This is because fluctuations in a resonance vibration frequency of the piezoelectric vibrator due to aging and ambient temperature variation are relatively small in comparison with other electronic parts. In addition, the piezoelectric vibrator exhibits a excellent short-term stability of the frequency, so that the piezoelectric vibrator is indispensable for operating the oscillation circuit and using electronic devices. A piezoelectric property of a piezoelectric material and a resonance frequency characteristic of the piezoelectric vibrator are extremely useful for the filter too.
A voltage-controlled piezoelectric oscillation circuit is largely employed as a TCXO (temperature compensated crystal oscillator) for controlling a reference frequency of a portable telephone and a timing-recovering element of a digital circuit. The voltage-controlled piezoelectric oscillation circuit typically has a frequency controlling function. A frequency of the voltage-controlled piezoelectric oscillation circuit is typically controlled by a variable-reactance element such as a variable-capacitance diode.
Operation frequencies of a typical piezoelectric vibrator vary in a range from several kHz up to several tenth GHz, and thus the piezoelectric vibrator generates a signal whose frequency is adjusted over such the wide range. Depending on the frequency, vibrational motions of the piezoelectric vibrator are referred to as, for example, a tuning fork vibration, a bending vibration, a longitudinal (extensional) vibration, a face shear vibration, a thickness shear vibration, surface wave vibrations, including a face shear, a coupling mode vibration, and a Stoneley surface wave.
Recently, piezoelectric resonators referred to as a SMR (Solid Mount Resonator) and a FBAR (Film BuIk Acoustic Resonator) are proposed. A piezoelectric device (see, for example, “Technical Handbook of Surface Acoustic Wave Device” edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., 1991 and “Technical Handbook of Surface Acoustic Wave Device” edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., 2004) utilizing a MEMS (Micro Electro Mechanical System) technology is also proposed. A new type resonator having interleaved electrodes so as to excite Lamb waves of a high frequency (see, for example, Japanese Patent No. 3400165) is also proposed.
However, low electric power consumption and miniaturization of the oscillation circuit are prevented by the variable-capacitance diode for controlling a frequency thereof.
For the sake of expanding a variable-frequency range of the oscillation circuit, it is required to increase a variation width of the capacitance value of the variable-capacitance diode. However, the variation width of the variable-capacitance diode depends on a variation width of an applied voltage thereon, thus it is necessary to increase the applied voltage. A requirement for expanding the variable-frequency range conflicts to a requirement for decreasing the applied voltage of the oscillation circuit. Therefore, both of lowering a power supply voltage effective for low power consumption and an IC integration for miniaturization are not compossible.
For the purpose of decreasing power supply voltage, a super-cascade type variable-capacitance diode having a large variation width of a capacitance value is used as the variable-capacity diode. In a present production-line, the variable-capacitance diode of this type together with other circuit parts can not be integrated into an IC device for the sake of miniaturization. Therefore, there is no other choice to product an oscillation circuit by assembling the variable-capacitance diode as an individual circuit part.
Moreover, a mean of precisely controlling a frequency over the wide rage is useful in not only an oscillation circuit but also a filter and various resonance circuits. A new frequency controlling mean replaced by variable-capacitance diode is demanded.
The present invention is directed to solving the problems described above and provides a resonance circuit that can eliminate a variable-reactance element and a variable-inductance element, both of which are utilized in a typical resonance circuit, and also can control an oscillation frequency of a piezoelectric resonator and a frequency characteristic of a filter. The present invention also provides an oscillation circuit and a filter using the resonance circuit. The present invention also provides a complex resonance circuit including a piezoelectric vibrator whose frequency can be adjusted over a wide rage beyond a limitation of a variable-frequency width. The variable-frequency is dependent upon vibrational motion of the piezoelectric vibrator.